A. Field of the Invention
The invention relates to a method and an apparatus for checking value documents, such as e.g. bank notes, checks, cards, tickets, coupons.
From the prior art it is known to provide value documents with security elements, such as security strips or also security threads, which contain magnetic material. The magnetic material here can be applied onto the security element either continuously or only in certain regions, for example in the form of a coding. For the magnetic coding of a security element there is used for example a certain sequence of magnetic and non-magnetic regions, which is characteristic of the value document. Furthermore, it is known to employ different magnetic materials for a magnetic coding, e.g. with different coercive field strengths. In the hitherto known magnetic codings there are used two differently coercive magnetic materials, of which there are formed two types of magnetic regions which are disposed on the security element.
Further, it is known to automatically check bank notes having security threads, which have a magnetic coding of differently coercive materials. In so doing, the bank notes are transported parallel to the longitudinal direction of the security element, and one after the other first run through a strong magnetic field parallel to the transport direction, which magnetizes both the high-coercive and also the low-coercive magnetic regions along the transport direction. The remaining magnetization is checked by means of an inductive magnetic detector which is sensitive parallel to the transport direction. Then the bank notes run through a weaker magnetic field perpendicular to the transport direction, which aligns only the low-coercive magnetic regions perpendicular to the transport direction, while the high-coercive magnetic regions remain magnetized in the transport direction. Anew, the remaining magnetization is checked by means of an inductive magnetic detector which is sensitive parallel to the transport direction. In so doing, with the first inductive magnetic detector the high- and the low-coercive magnetic regions are detected and with the second inductive magnetic detector only the high-coercive magnetic regions are detected. If the security element, however, also contains combined magnetic regions, which contain the two differently coercive magnetic materials, so that the differently coercive magnetic materials at the same time reach the detection region of the magnetic detector, an overlay of the magnetic signals of the differently coercive magnetic materials is detected. The combined magnetic regions deliver here a reduced magnetic signal whose signal swing lies between that of the high-coercive and that of the low-coercive magnetic regions. A disadvantage of this method is that these combined magnetic regions can be distinguished only with difficulties from the high-coercive and from the low-coercive magnetic regions. Furthermore it is disadvantageous to employ for the magnetic reversal of the low-coercive magnetic regions a magnetic field which is oriented parallel to the transport plane and perpendicular to the transport direction of the value document. Because with the aid of usual magnets such oriented magnetic fields can only be realized with relatively low magnetic field strength.